Jones D.S.J., Pujado P.R. Handbook of Petroleum Processing
Подождите немного. Документ загружается.
946 CHAPTER 18
Table 18.20 summarizes the application of the various sealing systems in the refinery
operation.
Pump drivers and utilities
Most pumps in the process industry are driven either by electric motors, or by steam,
usually in the form of steam turbines. This item deals with the calculation of the driver
requirements and its specification.
Electric motor drivers
Electric motors are by far the most common pump driver in industry. They are more
versatile and are cheaper than a comparable size of steam turbine. The electric motors
used for pump drivers are the induction type motor. They range in size from fractional
horsepower to 500 and higher horsepower. Sizing the required motor for a pump driver
takes into consideration the pump brake horsepower, the energy losses occurring in
the coupling device between the pump and the motor, and a contingency factor of
about 10%. These are expressed by the equation:
Minimum driver BHP =
Maximum pump BHP × 1.1
Mechanical efficiency of coupling
If the pump is driven through a direct coupling the efficiency will be 100%. With
gears or fluid coupling the efficiency will be between 94% and 97%.
Specifying motor driver requirements
Process engineers are called upon very often to specify pump driver requirements or
to check the already existing. In doing this two items of data need to be obtained or
calculated. These are:
r
The actual required horsepower of the pump motor to drive the pump at its specified
duty
r
What is actually installed in terms of horsepower
These data are tabulated in terms of power load as follows:
Operating load, kW—This is power input to the motor a normal operating horsepower.
Connected load, kW—This is power input to the motor at motor rated horsepower.
If the pump is spared by another motor driven pump then the connected load will be
the sum of both motors.
Operating load =
Minimum required driver HP × .746
Efficiency of the motor @ its operating HP
Connected load =
Rated motor HP × .746 × number of motors
Efficiency of the motors @ 100% Full load
.
PROCESS EQUIPMENT IN PETROLEUM REFINING 947
Table 18.20. Application of pump sealing systems
948 CHAPTER 18
Table 18.21. Electric motor size and efficiency
Motor efficiency
ata%offull load
Motor Motor
rating connected load,
BHP kW 50 75 100
1 0.98 68 74 76
1.5 1.42 72 76.5 79
2 1.86 73 78 80
3 2.76 77.5 81.5 82
5 4.39 83 82 85
7.5 6.65 81 83.5 84
10 8.78 84 85 85
15 13.0 85 86 86
20 17.05 86.5 87.5 87.5
25 21.0 87.5 88.5 88.5
30 25.1 88 89 89
40 33.5 88 89 89
50 41.7 88 89.5 89.5
75 62.1 89 90 90
100 82.0 84 89 91
125 102.0 85 89.5 91.5
150 123.0 86 89 91
200 161.0 88 91 92.5
250 201.0 90.5 92.5 92.5
300 241.0 90.5 92.5 92.8
350 281.0 90.9 92.6 92.9
400 320.0 91.1 92.8 93.1
450 360.0 91.2 93 93.2
Table 18.21 gives the motor sizes, and standard efficiencies at % of full load. (Higher-
efficiency and variable speed motors may also be available.)
Example calculation
Calculate the operating and connected loads for pump 11-P-3 A&B as specified in
Figure 18.19 of this chapter.
From the pump calculation sheet the hydraulic horsepower is calculated:
HHP =
lbs/min × diff head in feet
33,000
=
8,665 × 264.3
33,000
= 69.4HP
From Figure 18.19 and assuming 60 cycle pump speed pump efficiency is 79%
Then brake horsepower =
Hydraulic Horsepower
.79
= 87.8HP
PROCESS EQUIPMENT IN PETROLEUM REFINING 949
This will be a direct driven pump thus coupling efficiency is 100%
Minimum motor size = BHP × 1.1 = 96.6HP
The closest motor size to this requirement is 100 HP (Table 18.21). This is a little too
close so a motor size of 125 HP will be selected.
Operating Load =
Rated HP × 0.746
Efficiency@%offull load.
% of Full load =
87.8
125
= 70.2%
Efficiency = 89% (from Table 18.21)
Operating load =
87.8 × .746
0.89
= 73.6kW
Connected load =
125 × .746
0.915
= 101.9 kW say 102 kW
Note if both regular and spare pumps were motor driven then the connected load
would be 2 × 102 = 204 kW.
Reacceleration requirement
To complete the specification for the motor requirement a degree of process impor-
tance of the pump must be established and noted. Voltage drops that can occur in any
system may be sufficient to stop the pump. The process engineer must determine how
important it is to the process and the safety of the process to be able to restart and
reaccelerate the particular pump quickly. The following code of importance has been
adopted:
r
Re acceleration absolutely necessary.—A
r
Re acceleration desirable.—B
r
Re acceleration unnecessary.—C
The ‘A’category involves any pump critical to keeping the process on stream safely
and with no possibility of equipment damage. The ‘B’category applies to those
pumps that in operation with the ‘A’category will maintain the unit ‘on spec’.
The ‘C’category refers to those pumps that can be started manually without any
problems.
950 CHAPTER 18
In the case of the example pump 11-P-3 A&B given here, the service required of the
pump is so critical to the operation and orderly shutdown of the process in the case
of power failure that the spare pump is Turbine driven. Thus the motor driven regular
pump need only be coded ‘C’for reacceleration.
Steam turbine drivers
Steam turbines are the second most common pump drivers in modern day process
industry. Although more expensive than the electric motor they offer an excellent
stand-by to retain the maximum process ‘on stream’time. The one big disadvantage
with power driven pumps is the reliability of power availability. Steam turbines there-
fore offer a good alternative in cases of power failure. Another alternative means
of pump drivers is the diesel engine or gas engine, but these require their own fuel
storage etc. and are certainly not as reliable as the steam turbine.
Most process plants therefore spare the critical pumps in the process with a turbine
driven unit which may be started automatically on low process flow.
The principle of the turbine driver
Turbines are the most flexible of prime movers in today’s industry. Their horsepower
output can be varied by the number and size of the steam nozzles used, speeds can be
changed readily, and high speeds without gearing are possible. They have a very wide
range of horsepower applications. The operation of the steam turbine is analogous to
that of a water wheel where buckets are attached to the wheel which collect the water.
The wheel is moved downwards by the weight of the water collected and thus cause
the rotation of the wheel. In steam turbines the buckets are replaced by vanes which
are impinged by the motive steam to cause the rotating motion. Turbines may consist
of one set of vanes keyed to the shaft in the case of a single stage machine or several
sets of vanes in the case of multi stage machines. These sets of vanes are called simply
‘wheels’and the number of stages are referred to as the number of wheels.
In the case of multistage turbines, the steam leaving the first wheel is directed towards
a set of stationary vanes attached to the casing. These stationary vanes reverse the
steam flow and serve as nozzles directing the steam toward the second wheel attached
to the same shaft.
Most turbines used on a regular basis in a process plant are single stage. Multi stage
machines are more efficient but are also much more expensive. Their use therefore
is for drivers requiring horsepower in excess of 300. The power industry is a good
example for the use of large multi stage turbine drivers. Single or multi stage turbines
may be operated either condensing or non condensing. However pump drivers should
PROCESS EQUIPMENT IN PETROLEUM REFINING 951
not be made condensing without a rigorous review to see if other types of drives can
be used. The complexity of condensing is hardly worth the small savings in utilities
that are made.
The performance of the steam turbine
The salient factors in the performance of the steam turbine are:
r
Horsepower output
r
Speed
r
Steam inlet and outlet conditions
r
Its mechanical construction (e.g., number of wheels, size of the wheel, etc.)
These factors are interrelated and their effect on the performance of the turbine is
reflected by a change in over-all efficiency. The overall efficiency may be defined
as the ratio of the energy output to the energy of the steam theoretically available
at constant entropy as obtained from a Mollier diagram. This over-all efficiency
is the product of mechanical and thermal efficiencies. The losses in turbines are
partly due to friction losses of the rotating shaft and partly to thermodynamic losses
and turbulence. Figure 18.22 gives the overall efficiencies plotted against delivered
horsepower.
The steam required by a turbine for a given horsepower application is called its “Water
Rate”. The actual water rate for a turbine is supplied by the manufacturer from test
runs carried out on the actual machine in the workshop. Plant operators and other
engineers very often need to be able to estimate these water rates for their work. A
typical situation arises when determining the best steam balance for a plant. Such
estimates may be obtained from Figures 18.23a and b. This and the accompanying
notes are self explanatory.
Cooling water requirements
Many pumps in process service require water cooling to various parts of the pump.
This cooling water is applied to bearings, stuffing boxes, glands, and pedestals. The
application of the cooling water is determined by the manufacturer in accordance
with his standard for the service and conditions that the pump must satisfy. Most of
the cooling water may be recovered in a closed cooling water system. However gland
cooling water is never recovered but is routed to the waste water drain. The following
lists the approximate cooling water requirements for pumps and steam turbines:
Pumps To 1,000 gpm Above 1,000 gpm
Up to 350 F 0 gpm 0 gpm
350 F–500 F 2 gpm 4 gpm
Above 500 F 3 gpm 6 gpm
Figure 18.22. Steam turbine efficiencies.
PROCESS EQUIPMENT IN PETROLEUM REFINING 953
Figure 18.23. Water rates for condensing and non-condensing turbines.
954 CHAPTER 18
Steam Turbines.
450 F 0 gpm 0 gpm
Above 450 F 3 gpm 3 gpm
18.3 Compressors
Types of compressors and selection
Compressors are divided into four general types, these are:
r
Centrifugal
r
Axial
r
Reciprocating
r
Rotary
The name given to each type is descriptive of the means used to compress the gas and
comparison of the different types of compressors and typical applications is shown
in Table 18.22. A brief description of each of the types now follows:
Centrifugal
This type of compressor consists of an impeller or impellers rotating at high speed
within a casing. Flow is continuous and inlet and discharge valves are not required
as part of the compression machinery. Block valves are required for isolation during
maintenance.
Centrifugal compressors are widely used in the petroleum, gas, and the chemical
industries primarily due to the large volumes of gas that frequently have to be handled.
Long continuous operating periods without an overhaul make centrifugal compressors
desirable for use for petroleum refining and natural gas applications. Normally they
are considered for all services where the gas rates are continuous and above 400
ACFM (actual cubic feet per minute) for a clean gas, and 500 ACFM for a dirty gas.
These rates are measured at the discharge conditions of the compressor. Dirty gases
are considered to be gases similar to those from a catalytic cracker, which may contain
some fine particles of solid or liquid material.
The slowly rising head-capacity performance curves make centrifugal compressors
easy to control by either suction throttling or variable speed operation.
The main disadvantage of this type compressor is that it is very sensitive to gas density,
molecular weight and polytropic compression exponent. A decrease in density or
PROCESS EQUIPMENT IN PETROLEUM REFINING 955
molecular weight results in an increase in the polytropic head requirement of the
compressor to develop the required compression ratio.
Axial flow
These compressors consist of bladed wheels that rotate between bladed stators.
Gas flow is parallel to the axis of rotation through the compressor. Axial flow com-
pressors become economically more attractive than centrifugal compressors in ap-
plications where the gas rates are above 70 000 ACFM at suction conditions. The
compressors are extremely small relative to capacity and have a slightly higher effi-
ciency than the centrifugal. Axial flow compressors are widely used as air compressors
for jet engines and gas turbines.
Reciprocating
Reciprocating compressors are widely used in the petroleum and chemical industries.
They consist of pistons moving in cylinders with inlet and exhaust valves. They are
cheaper and more efficient than any other type in the fields in which they are used.
Their main advantages are that they are insensitive to gas characteristics and they
can handle intermittent loads efficiently. They are made in small capacities and are
used in applications where the rates are too small for a centrifugal. Reciprocating
compressors are used almost exclusively in services where the discharge pressures
are above 5,000 psig.
When compared with centrifugal compressors, the reciprocating compressors require
frequent shutdowns for maintenance of valves and other wearing parts. For critical
services this requires either a spare compressor or a multiple compressor installation
to maintain plant throughput. In addition, they are large and heavy relative to their
capacity.
Rotary
Recent developments in the rotary compressor field have opened up areas of appli-
cation in the process industry with the use of the following types of rotary compressors:
(1) High pressure screw
These compressors have been developed into heavy duty type machines. They
consist of two rotating helices that rotate in a casing without actual contact.
Rotary compressors are lower in cost and have a higher efficiency than centrifugal
compressors. They are not sensitive to gas characteristics since they are positive
displacement machines. Parts are standardized production items so that a spare
rotor is not generally required to be stocked for emergency replacement.